import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int col = in.nextInt();
        int row = in.nextInt();
        int count = 0;
        int[][] array = new int[row][col];
        for (int i = 0; i < row; i++) {
            for (int j = 0; j < col; j++) {
                /*二维数组中默认值为0，每在一个元素为0的位置放一块蛋糕（然后计数器+1）
                * 就将不能放置蛋糕的位置置为1*/
                if (array[i][j] == 0){
                    count++;
                    //判断i+2是否越界
                    if (i+2 < row){
                        array[i+2][j] = 1;
                    }
                    //判断j+2是否越界
                    if (j+2 < col){
                        array[i][j+2] = 1;
                    }
                }
            }
        }
        System.out.println(count);
    }
}
